Compared with a two-stroke engine, a four-stroke engine has the advantages of cleaner exhaust and fuel saving. As such, more and more light duty, general industrial/agricultural machines adopt a four-stroke engine according to the Regulation on Environmental Protection. At the present, there are two types of lubricating devices used for a four-stroke engine, i.e., a wet (wet sump) and a dry type. A difference between them is whether or not a common space is used for the crankcase and the oil box.
FIG. 1 shows a design of American RYOBI Co. published in SAE TECHNICAL PAPER SERIES 961728, which is a common wet lubricating device for a four-stroke engine. In this device, a crankcase 11 and an oil box of the four-stroke engine have a common space, i.e., it is a so-called wet sump. This device can be used with certain light duty tools, such as a grass cutter, lopping shears, wood saw and so on. As will be appreciated, these tools are widely used and operated on various inclinations. However, the engine 10 is restricted to operating on no more than a slight inclination to avoid oil flowing out from the oil box. If the engine operates on a larger inclination, oil will flow out from the oil box to the gap between the piston 13 and the cylinder wall and will be pumped into the combustion chamber, resulting in engine failure or oil flowing into the cylinder head and then out from the vent/filter.
In order to allow the engine mentioned above to operate on any inclination, conventional designs also adopt a fully separated oil box, i.e. the oil box and the crankcase are completely separate, to form a so-called dry sump. FIG. 2 shows a design of Japanese Patent of Application No. 7-327665, Application date of Dec. 15, 1995, in which a proper amount of oil is pumped to points to be lubricated in the engine, or oil 22 is splashed by a slinger 21 to create an oil fog which is pumped to the crankcase 23 and the cylinder head 25 for lubrication using suction generated by the piston 24. Although the dry sump has a better structure than the wet sump, an additional oil trough, pressure regulating devices and additional drillings in the crankshaft are needed, resulting in a more complex structure and higher cost.